1. Field of the Invention
The present invention relates to a method of crystallization, and more particularly, to a method of deciding a focal plane and a method of crystallization using the focal plane deciding method, which improve the crystallization process by deciding a best-fit focal plane using a test mask and then applying the decided best fitted focal plane to the crystallization.
2. Discussion of the Related Art
Recently, as the interest in an information display has been increased and there is a continually increasing demand for portable information media, researches on thin film type Flat Panel Display (FPD) devices, which are replacing Cathode Ray Tubes (CRTs), and its commercialization have been made preponderantly. In particular, a Liquid Crystal Display (LCD) device among such flat panel display devices displays images using an optical anisotropy of liquid crystal. The LCD device has been actively applied to notebook computers, desktop monitors, or the like because it has an excellence in resolution, color rendering capability and picture quality.
An Active Matrix (AM) driving method, a driving method mainly used in the LCD device, drives the liquid crystal of a pixel region using an amorphous silicon Thin Film Transistor (TFT) as a switching device.
In 1979, English LeComber established conception of amorphous silicon thin film transistor technology. In 1986, it was put to practical use as a three-inch liquid crystal portable television. Lately, a TFT-LCD device with a large display area of more than 50 inches has been developed.
However, with the field effect mobility (<1 cm2/Vsec) of the amorphous silicon thin film transistor, there is a limit when the amorphous silicon thin film transistor is used in peripheral circuits that require high-speed movement of more than 1 MHz. Accordingly, researches for simultaneously integrating a pixel region and a driving circuit region on a glass substrate using a polycrystalline silicon thin film transistor of which field effect mobility is greater than that of the amorphous silicon thin film transistor have been actively pursued.
The polycrystalline silicon thin film transistor technology has been applied to small modules such as a camcorder or the like since an LCD color television was developed in 1982. Since it has low photosensitivity and high field effect mobility, it can be directly fabricated on a substrate to form driving circuits.
Increased mobility can increase the operation frequency of the driving circuit unit for determining the number of driving pixels, which facilitates fixing of the display device. Also, the picture quality could increase since the distortion of the transfer signal is decreased because of the reduction in the charging time of a signal voltage of the pixel unit.
In addition, since the polycrystalline silicon thin film transistor can be driven at less than 10V in comparison with the amorphous silicon thin film transistor having a high driving voltage (˜25V), it has an advantage that the power consumption can be reduced.
Meanwhile, the polycrystalline thin film transistor can be mainly fabricated by directly depositing a polycrystalline silicon thin film on a substrate and by depositing an amorphous silicon thin film that is then crystallized by a thermal process. In particular, to use a cheap glass as a substrate, low temperature processes are required, and, to use the polycrystalline silicon thin film for a driving circuit unit, a method for increasing the field effect mobility of the thin film transistor is required.
Thermal processing methods for crystallizing an amorphous silicon thin film are the Solid Phase Crystallization (SPC) method and the Excimer Laser Annealing (ELA) method, mainly.
The solid phase crystallization method forms a polycrystalline silicon thin film at a temperature of approximately 600° C. In this method, a polycrystalline silicon thin film is crystallized by forming an amorphous silicon thin film on a glass substrate and then by performing a heating process at approximately 600° C. for up to a few hours or tens of hours. A polycrystalline silicon thin film obtained by the solid phase crystallization has comparatively large-size grains of about several μm. However, there are many defects in the grains. Although not as bad as grain boundaries in a thin film transistor, it is known that these defects affect negatively on the performance of the thin film transistor.
The excimer laser annealing method is an essential method of fabricating a polycrystalline silicon thin film at a low temperature. The amorphous silicon thin film is crystallized by momentarily irradiating a high energy laser beam onto the amorphous silicon thin film for tens of nanoseconds. The amorphous silicon is melted and crystallized in a very short time, so that the glass substrate is not damaged.
In addition, a polycrystalline silicon thin film fabricated using the excimer laser has excellent electrical characteristics, compared to a polycrystalline silicon thin film fabricated by a general thermal processing method. For example, a field effect mobility of a polycrystalline silicon thin film transistor fabricated by the excimer laser annealing method is more than 100 cm2/Vsec, whereas a field effect mobility of an amorphous silicon thin film transistor is generally 0.1 to 0.2 cm2/Vsec and a field effect mobility of a polycrystalline silicon thin film transistor fabricated by a general thermal processing method is 10 to 20 cm2/Vsec (IEEE Trans. Electron Devices, vol. 36, no. 12, p. 2868, 1989).
Hereinafter, a crystallization method using a laser according to a related art will be described in detail.
FIG. 1 is a graph illustrating a grain size of a polycrystalline silicon thin film with respect to an energy density of a laser to be irradiated.
As shown in FIG. 1, in the first and second regions (I) and (II), as the energy density increases, the grain size of the polycrystalline silicon thin film increases (IEEE Electron Dev. Lett., DEL-7, 276, 1986). However, in the third region (III), when the laser having an energy density higher than a specific energy density Ec is irradiated, the grain size of the polycrystalline silicon thin film decreases drastically. That is, the crystallization mechanism for the silicon thin film becomes different according to the energy densities of the laser to be irradiated.
FIGS. 2A to 2C, 3A to 3C and 4A to 4C are sectional views illustrating silicon crystallization mechanisms according to the laser energy densities of the graph shown in FIG. 1. They illustrate sequential crystallization processes according to each laser energy density.
A crystallization mechanism of amorphous silicon by a laser annealing is influenced by various factors such as laser irradiation conditions including laser energy density, irradiation pressure, substrate temperature, and physical/geometrical characteristics including absorption coefficient, thermal conductivity, mass, impurity containing degree and thickness.
First, as shown in FIGS. 2A to 2C illustrating the crystallization process for the first region (I) of the graph in FIG. 1, since the first region (I) of the graph in FIG. 1 is a partial melting region, an amorphous silicon thin film 12 is crystallized only up to the dotted line and a size of a grain 30 formed at this time is about hundreds Å.
When a laser having the energy density in the first region (I) is irradiated on the amorphous silicon thin film 12 on a substrate 10 where a buffer layer 11 is formed, the amorphous silicon thin film 12 is melted. At this time, because strong laser energy is irradiated at a surface of the amorphous silicon thin film 12, which is directly exposed to the laser beam, and relatively weak laser energy is irradiated at a lower portion of the amorphous silicon thin film 12, a certain portion of the amorphous silicon thin film 12 is melted to form a melted portion 12′. As a result, crystallization is partially performed.
Typically, in the crystallization method, crystal grows through the processes of primary melting in which a surface layer of an amorphous silicon thin film is melted by a laser irradiation, and secondary melting in which a lower portion of the amorphous silicon thin film is melted by the latent heat generated during the solidification of the melted silicon and the solidification of the lower layer. These crystal growth processes will be explained in more detail.
An amorphous silicon thin film on which a laser beam is irradiated has a melting temperature of more than 1000° C. and is primarily melted into a liquid state. Because there occurs a great temperature difference between the primarily melted layer and the lower silicon and substrate, the primarily melted layer cools fast until solid phase nucleation and solidification are generated. The melted layer by the laser irradiation is maintained until the solid phase nucleation and solidification take place. Such a melting state lasts in a range that ablation does not occur for a long time when the laser energy density is high or thermal emission to the outside is low. In addition, because the primarily melted layer melts at a temperature of 1000° C. lower than the melting temperature of 1400° C. for crystalline silicon, the melted layer cools and maintains a super-cooled state where the temperature is lower than the phase transition temperature. The greater the super-cooling state is (that is, the lower the melting temperature of the thin film or the faster the cooling is), the greater the nucleation rate is at the time of the solidification such that fine crystals grow during the solidification.
When the solidification starts as the primarily melted layer cools, crystals grow in an upward direction from a crystal nucleus. At this time, latent heat is emitted by the phase transition of the primarily melted layer from the liquid state to the solid state, and thus the secondarily melting begins where the lower amorphous silicon thin film of solid state melts. Such processes are repeated through the solidification whereby the crystals grow. At this time, the nucleus generation rate of the lower secondarily melted layer increases, because the lower amorphous silicon thin film is more super-cooled than the primarily melted layer. Thus, the crystal size resulting from the secondarily melted layer is smaller.
Accordingly, in crystallization by a laser annealing, the cooling speed of the solidification has to be reduced to improve the crystalline characteristics. Cooling speed can be reduced by restraining absorbed laser energy from being emitted to the outside by using the restraining methods such as heating the substrate, a double beam irradiation, or inserting a buffer insulating layer.
FIGS. 3A to 3C are sectional views sequentially illustrating the silicon crystallization mechanism for the second region (II) of the graph in FIG. 1, in which the second region (II) represents a near-complete melting region.
As show in FIGS. 3A to 3C, a polycrystalline silicon thin film has relatively large grains 30A to 30C of about 3000 to 4000 Å formed down to the interface of the lower buffer layer 11.
Namely, when a nearly complete melting energy, not a complete melting energy, is irradiated on the amorphous silicon thin film 12, almost all of the amorphous silicon thin film 12 down close to the buffer layer 11 melts. At this time, there exist solid seeds 35 that have not been melted at the interface between the melted silicon thin film 12′ and the buffer layer 11 (FIG. 3A). The solid seeds 35 work as a crystallization nucleus to induce side growth, thereby forming the relatively large grains 30A to 30C (J. Appl. Phys. 82, 4086).
However, since such crystallization method is possible only if the laser energy is irradiated such that the solid seeds 35 that are not melted remain on the interface with the buffer layer 11, the process window is very narrow. In addition, because the solid seeds 35 are generated non-uniformly, the crystallized grains 30A to 30C of the polycrystalline silicon thin film have different crystalline directions, that is, they have non-uniform crystallization characteristics.
Finally, FIGS. 4A to 4C are sectional views illustrating the crystallization mechanism of the third region (III) of the graph in FIG. 1 corresponding to a complete melting region.
As shown therein, very small grains 30 are irregularly formed with an energy density corresponding to the third region (III).
That is, when the laser energy density becomes higher than a specific energy density Ec, sufficient energy is applied enough to completely melt the amorphous silicon thin film 12, leaving no solid seeds that may be grown to grains. Thereafter, the silicon thin film 12′ which has been melted upon receiving the strong energy laser undergoes a rapid cooling process, which generates a plurality of uniform nuclei 135 and thus fine grains 30.
Crystallization in that range has an advantage that the process window is large, but has a disadvantage that the grains 30 of the crystallized silicon thin film are fine.
Meanwhile, since a polycrystalline silicon thin film having different crystallization characteristics according to the energy density of the laser to be irradiated is formed, in order to obtain a crystalline silicon thin film having desired characteristics, it is important to control a laser such that the laser to be irradiated on a substrate has a best-fit focal plane.
FIG. 5 is an exemplary view illustrating a variety of focal planes of the laser beam. As shown therein, (A), (B) and (C) indicate a deep focus, a just focus and an under focus, respectively, in which the focal planes are formed at a lower portion layer, a surface and the outside of a substrate 210, respectively.
At this time, in case that the crystallization is performed by the just focus (B) in which the focal plane of a laser beam 260 through a lens 265 is formed on a surface of the substrate 210 on a stage 240, a crystallized thin film having the clearest crystallization shape can be obtained, whereas in case of the deep focus (A) and the under focus (C), a thin film having a clear crystallization shape cannot be obtained.
That is, in the laser crystallization, forming of a clear crystallization shape means that laser energy is made incident enough to melt an amorphous silicon thin film. In order to perform optimum crystallization processes, it is very important for a focal plane state of the laser beam to be irradiated to make the above mentioned just focus.
However, up to now, the crystallization is performed without taking a focal plane of a laser beam into account, crystallization characteristics of the crystallized silicon thin film are observed, and laser equipment and optical system are compensated, before carrying out the crystallization processes. As a result, there have been problems in that the time for the crystallization processes and the cost increase.
In addition, the above mentioned problems happen when a new mask for crystallization process is loaded onto a mask stage and when the laser equipment and optical systems are re-set.